1. Field of the Invention
The present invention relates to a dial gauge. More specifically it relates to a dial gauge for detecting a displacement of a spindle to measure a dimension of a workpiece.
2. Description of Related Art
A dial gauge 1 shown in FIGS. 7 and 8 is known as a measuring instrument for measuring a dimension of a workpiece.
The dial gauge 1 has a spindle 3 having a probe 2 to be in contact with the workpiece at a distal end thereof, a body 4 having a display 4B for displaying a measured value by a pointer 4A, and a cylindrical stem 5 provided to the body 4 for slidably holding the spindle 3 through a stem bush 10, the dimension of the workpiece is measured by detecting a displacement of the spindle 3 relative to the body 4. Among the above components, the stem bush 10 is formed in a cylindrical shape having outer circumference being fixed to an inner wall of the stem 5 and inner circumference slidably holding the spindle 3. A predetermined clearance is formed between the inner circumference of the stem bush 10 and the outer circumference of the spindle 3. The stem bush 10 is located on the probe 2 side of the stem 5.
The dial gauge 1 sometimes is used to measure the workpiece while being mounted to a supporting stand 6 as shown in FIG. 7.
The supporting stand 6 has a base 6B having a measuring table 6A on an upper surface thereof, a column 6C standing of the base 6B, and a bracket 6D vertically movable along the column 6C with the dial gauge 1 held thereon. The bracket 6D has a clamp hole 61 for the stem 5 of the dial gauge 1 to be inserted, a slitting groove 62 extending from the clamp hole 61 to an end (an end remote from the column 6C) of the bracket 6D, and a clamp screw 63 for elastically deforming the end of the bracket 6D to narrow the width of the slitting groove 62.
In use, after the stem 5 of the dial gauge 1 is inserted to the holding hole 61 of the bracket 6D, the dial gauge 1 is fixed to the bracket 6D by clamping the clamp screw 63. At this time, a clamp position 5A (outline arrow in FIG. 8) of the stem 5 by the clamp screw 63 is remote from the stem bush 10 by a predetermined distance along an axial direction of the spindle 3. In this state, after the spindle 3 of the dial gauge 1 is once lifted, the workpiece is set on the measuring table 6A. Subsequently, the spindle 3 is lowered so that the probe 2 abuts the workpiece. The height of the workpiece is measured by reading the dial gauge at the time.
However, since the above-described dial gauge 1 is mounted to the bracket 6D of the supporting stand 6 by clamping the stem 5 with the clamp screw 63, the stem 5 can be slightly deformed on account of the clamp force of the clamp screw 63 for clamping the stem 5. When the stem 5 is deformed, the stem bush 10 fitted to the inner wall of the stem 5 also deforms. The deformation of the stem bush 10 can bring the inner circumference of the stem bush 10 into contact with the outer circumference of the spindle 3, so that the inner circumference of the stem bush 10 holds the outer circumference of the spindle 3, thus deteriorating slidability of the spindle 3.
In order to overcome the above disadvantage, the clearance between the spindle 3 and the stem bush 10 may be set wide. However, when the clearance is enlarged, shaky movement of the spindle 3 relative to the stem bush 10 can be magnified, so that the measurement accuracy can be deteriorated.
According to an aspect of the present invention, a dial gauge includes: a spindle having a probe to be in contact with a workpiece at a distal end thereof; a body; and a stem provided to the body for slidably holding the spindle through a stem bush, the stem bush having: a fitting fitted to an inner wall of the stem while securing a predetermined clearance against the spindle; a holder for slidably holding the spindle, and having a predetermined clearance against the inner wall of the stem; and an elastically deformable connector for connecting the fitting and the holder, the clearance between the fitting and the spindle being larger than the clearance between the holder and the spindle.
When the dial gauge is mounted to the supporting stand by clamping the stem, the stem can be deformed by the clamping force. Since the clearance between the fitting and the spindle is set large in the stem bush, the inner circumference of the fitting is not in contact with the outer circumference of the spindle even when the fitting secured to the inner wall of the stem is deformed in accordance with deformation of the stem. On the other hand, since the clearance between the stem and the holder is large, the inner wall of the deformed stem is not in contact with the outer circumference of the holder. In other words, the inner circumference of the holder is not pressed against the outer circumference of the spindle by the stem.
Further, since the fitting and the holder of the stem bush is connected by the elastically deformable connector, even when the fitting is deformed in accordance with the deformation of the stem, most of the deformation is absorbed by the elastic deformation of the connector, so that the deformation of the fitting is not directly transferred to the holder, thus minimizing the deformation of the holder. In other words, deterioration of slidability of the spindle relative to the stem bush can be minimized. Accordingly, even when the stem is deformed in fixing the dial gauge to the supporting stand, the deterioration of slidability of the spindle relative to the stem bush can be prevented.
Further, since the deformation of the holder in accordance with deformation of the stem is smaller than the conventional arrangement, the clearance between the holder and the spindle can be made smaller than the conventional arrangement without influencing on the slidability of the spindle. Accordingly, the shaky movement of the spindle and the holder can be made smaller than the conventional arrangement, thus improving the measurement accuracy.
In an arrangement of the present invention, the fitting may preferably be formed in a cylinder, and the holder may preferably be formed in a cylinder having a diameter corresponding to a diameter of the spindle.
Accordingly, since the fitting is cylindrically formed, bonding area of the fitting against the inner wall of the stem can be enlarged, thus securely fixing the fitting to the inner wall of the stem. Further, since the holder is cylindrical, the slide movement of the spindle in axial direction can be stabilized.
In an arrangement of the present invention, the fitting and the holder may preferably be spaced apart along an axial direction of the spindle, and the connector may preferably be a tapered cylinder connecting the fitting and the holder, one end of the connector having a diameter corresponding to the diameter of the fitting and the other end of the connector having a diameter corresponding to the diameter of the holder.
Accordingly, since the fitting and the holder are connected by the cylindrical connector having diameters of both ends corresponding to the diameter of the fitting and the holder respectively, the fitting can be strongly connected to the holder.
In the above arrangement, the holder may preferably located in the cylinder of the fitting, and the connector may preferably connect mutually adjacent ones of ends of the fitting and the holder, the thickness of the connector being smaller than the respective thickness of the fitting and the holder.
Accordingly, since the holder is provided in the cylinder of the fitting, the stem bush of the present invention can be used to a stem of short length. Further, since the thickness of the connector is smaller than respective thickness of the fitting and the holder, the connector is more easily deformed than the fitting and the holder, thus more effectively absorbing the deformation of the fitting by the connector.
In an arrangement of the present invention, the fitting and the holder may preferably be spaced apart along an axial direction of the spindle, and the connector may preferably connect a part of mutually opposing ends of the fitting and the holder, the connector being thinner than the respective cylinders of the fitting and the holder.
Accordingly, since a part of mutually opposing ends of the fitting and the holder is connected by the connector, the size of connecting portion of the fitting and the holder can be reduced, thus reducing transmission of the deformation of the fitting toward the holder. Further, since the connector is thinner than the fitting and the holder, the connector is more easily deformed than the fitting and the holder, thus more effectively absorbing the deformation of the fitting by the connector.